Extraframework aluminum species in zeolites: Ab initio molecular dynamics simulation of gmelinite

The dynamical behavior of extraframework aluminum particles (EFAL) in zeolites has been investigated using a first-principles molecular dynamics technique. Small clusters of hydrated aluminum hydroxide Al(OH)(3)(H2O)(3) and Al(OH)(3)(H2O) are located both in the main channel and in the cage. The simulation of the hexacoordinated cluster at T = 300 K shows an overcoordination of the central atom and a release of H2O molecules. When placed in the main channel both the EFAL and the noncoordinated H2O molecules are mobile. When placed in the cage, a network of hydrogen bonds is established, occluding the EFAL and suppressing its mobility. The basic character of the EFAL causes an exchange of Bronsted acid protons between the zeolite and the EFAL particle. In a high-acidity zeolite a multiple, simultaneous proton exchange is observed, leading to a separation of charges (EFAL(2+), zeolite(2-)). The calculated stretching OH frequencies of the EFAL are similar to60 cm(-1) higher than those of the Bronsted OH, in good agreement with IR measurements. (C) 2002 Elsevier Science (USA).